What is the Biblical basis for the belief that there are humans in heaven now?

It is very possible that there are human beings who have already lived in heaven with God -

Genesis 5:24 - "Enoch walked faithfully with God; then he was no more, because God took him away".

There is no mention of Enoch dying a physical death:

Hebrews 11:5 - "By faith Enoch was translated that he should not see death."

As for the criminal on the cross who was crucified next to Jesus, the verse simply states -

Luke 23:43 - "And Jesus said unto him, Verily I say unto thee, this day shalt thou be with me in paradise."

As for the mentioned verse, it actually depends where the comma is placed and seeing as in Latin there is no comma it may well be read; "I say to you this day, you will be with me in paradise". Notice the difference when a comma is added where it shouldn't be?

It does not necessarily mean that the two of them will be going to heaven on that exact day - because Jesus did not ascend to heaven until after his resurrection which would not happen until three days later!

John 20:17 - "Jesus said, 'Do not hold on to me, for I have not yet ascended to the Father."

The aforementioned verse takes place 3 days AFTER Jesus has died on the cross - clearly at that time Jesus dies he did not go back up to heaven and so neither could have the criminal. Jesus did not ascend to heaven until the third day after his death.

1 Corinthians 15:4 - "He was buried, and he was raised from the dead on the third day, just as the Scriptures said."

As for Elijah, you could look at 2 Kings 2:11 (ESV):

And as they still went on and talked, behold,chariots of fire and horses of fire separated the two of them. And Elijah went up by a whirlwind into heaven.

You didn't mention Mary specifically, but she may be of interest. Revelation 12:1-6 (ESV) may refer to her:

 And a great sign appeared in heaven: a woman clothed with the sun, with the moon under her feet, and on her head a crown of twelve stars. 2 She was pregnant and was crying out in birth pains and the agony of giving birth. 3 And another sign appeared in heaven: behold, a great red dragon, with seven heads and ten horns, and on his headsseven diadems. 4 His tail swept down a third of the stars of heaven and cast them to the earth. And the dragon stood before the woman who was about to give birth, so that when she bore her child he might devour it. 5 She gave birth to a male child, one who is to rule all the nations with a rod of iron, but her child was caught up to God and to his throne, 6 and the woman fled into the wilderness, where she has a place prepared by God, in which she is to be nourished for 1,260 days.

All Christians? Hardly so most of us do not believe that humans go to Heaven, what we believe is that our souls go to Heaven.

The hope of the Christian faith is that upon death, the soul goes to Heaven; that is based on our belief that Jesus paid our sin debt in full. There are several Scriptures in the Gospels which clearly define this phenomena.

Our hope is not that we will be physically perpetuated, but that there is a spiritual part of every human being which is eternal; that part is referred to as the soul.

Genesis 2:7 KJV  And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.

The dust is the physical part of the human being, while the soul is the perpetual part of the human; according to Christian belief, and is necessary if salvation is factual. Without an eternal part of the man as created by God, there could be no eternal punishment (or Spiritual death) nor such a thing as salvation.

It is basic to our faith that Jesus paid our sin debt in full when he perished on the cross. We believe that Jesus was deity and was without sin in his human person. That would personify the only acceptable sacrifice capable of assuming the sins of the World.

Now to your question in light of the above, Whether Enoch and Elijah were taken into Heaven in their physical form is contested in the Christian faith. Some believe that they were taken physically, while others think only their souls were taken into Heaven and their physical bodies discharged. Whether or not either is true is of no consequence to most Christians as it is acceptance into Heaven that is our driving force.

As far as the thief on the cross it is obvious that Jesus was referring to his soul as his physical being died on that cross and decayed. As for Moses the Bible says that God put his body away. However in the Gospels we are told that both Moses and Elijah visited with Jesus during his time on Earth.

Matthew 17:3 KJV  And, behold, there appeared unto them Moses and Elias talking with him.

These would have had to be Spiritual beings as several centuries had passed since their time on Earth. We as Christians accept these scriptures as valid, and assuming their validity, it is obvious that even though Elijah was taken up physically his soul is what is in existence in Heaven.

Christianity is a faith, and a faith is something believed without proof.

Whether or not you wish to accept that there are humans in Heaven, or that only souls are there it is first a necessity that you believe in an eternal part of the human being, and further that you believe in Heaven, which in turn means you believe in God as the Creator of all things and that Heaven is his abode. And if you believe these things you are required to believe in Jesus Christ as God's provision to perpetuate his creation for eternity in a form wholly acceptable to him. When you are at that state in Christian faith it is of no importance whether existence in Heaven is physical or Spiritual.

Hope this helps.

Genesis 5:24 certainly does substantiate that Enoch is still in heaven because there is mention that he was specifically ‘NO MORE’ on the earth for anyone to see ever again.

Whether or not his body is still alive in heaven is another question, Enoch could have died in the whereabouts God took him to. There is no indication as to whether or not his body received a transfiguration or not so that he could not die a mortal death. A transfiguration is when God gives a person an imperishable body: 1 Corinthians 15:52 – “In a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised imperishable, and we shall be changed.”

As for Philip, he was only moved to another place by the spirit so he could continue his work there - about 30 kilometres to the south from the Desert Road going south out of Jerusalem, where everyone saw him again. Acts 8:26 – “Now an angel of the Lord said to Philip, "Go south to the road--the desert road--that goes down from Jerusalem to Gaza… (39) Now when they came up out of the water, the Spirit of the Lord carried away Philip, and the eunuch saw him no longer. For he went his way rejoicing. (40) But Philip found himself at Azotus, and as he passed through he preached the gospel to all the towns until he came to Caesarea.”

The word heaven is used in different tenses and contexts throughout the Bible and heaven can means one of three things – The Sky, Outer Space or God’s Heavenly Place. The heavenly place is a special place in heaven that God has prepared for his people when they are taken and have received a transfiguration. John 14:2 – “In my Father’s house are many rooms. If it were not so, would I have told you that I go to prepare a place for you?”

Enoch may not have actually been to this part of heaven yet until Christ returns to earth for the first resurrection: Revelation 20:4 “…I saw the souls of those who had been beheaded for the testimony of Jesus and for the word of God… They came to life and reigned with Christ for a thousand years.”

When Jesus states no one has ascended to ‘heaven’ besides himself (John 3:13), he may be referring to the specially prepared placed that God has reserved for humans at the first resurrection where the people will be 'caught up’: 1 Thessalonians 4:17 – “Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air, and so we will always be with the Lord.”

Moses and Elijah may already have received a transfiguration considering they were seen to be alive and they were said to have died long ago (Deuteronomy 34:7 – “Moses was a hundred and twenty years old when he died.”). But they may not have yet visited that specially prepared place yet until the other saints are resurrected and have received the transfiguration. They may be working in the heavens with god until they receive the 'time of rest'.

Isaiah 66:1 – “Thus says the LORD: ‘Heaven is my throne, and the earth is my footstool; what is the house that you would build for me, and what is the place of my rest’?”

Hebrew 4:3 – “For we who have believed enter that rest, as he has said, ‘As I swore in my wrath, They shall not enter my rest’.”

In Revelation 4:4, we are introduced to "24 elders" who throw down their crowns (4:10) and worship God.

And round about the throne were four and twenty seats: and upon the seats I saw four and twenty elders sitting, clothed in white raiment; and they had on their heads crowns of gold.

There is a lot of speculation as to the identity of these gentlemen, however, and at what point in history are they there (are they there now, or do they arrive at a later date?). For instance, some commentators believe they are human beings from the church arriving there at a later date, or are there right now.

There are no explicit statements in the Bible that any of the people mentioned in the question are now in heaven. The belief on the part of many Christians that they are now in heaven is based on reading key Bible passages about their death and their status after death in light of a more generalized belief that humans go to heaven after they die.

On the people specifically mentioned in the question, here are the Bible passages commonly quoted to support a belief that they are now in heaven:

Enoch

Thus all the days of Enoch were three hundred sixty-five years. Enoch walked with God; then he was no more, because God took him. (Genesis 5:23-24)

And:

By faith Enoch was taken so that he did not experience death; and "he was not found, because God had taken him." For it was attested before he was taken away that "he had pleased God." (Hebrews 11:5)

Neither of these passages state that Enoch is in heaven now. But in light of the generalized belief among many Christians that people go to heaven after they die, these verses are read by such Christians as meaning that God took Enoch directly to heaven.

Moses & Elijah

These two are taken together because the Bible passages most commonly referred to as saying that they are now in heaven are the accounts of the Transfiguration of Jesus in the Synoptic Gospels. The full accounts are found in Matthew 17:1-13; Mark 9:2-8; Luke 9:28-36.

Here are the verses from these accounts in which Moses and Elijah are specifically mentioned, with the lead-in verse about the transfiguration of Jesus quoted only for the first one:

And [Jesus] was transfigured before them, and his face shone like the sun, and his clothes became dazzling white. Suddenly there appeared to them Moses and Elijah, talking with him. Then Peter said to Jesus, "Lord, it is good for us to be here; if you wish, I will make three dwellings here, one for you, one for Moses, and one for Elijah." (Matthew 17:2-4)

And there appeared to them Elijah with Moses, who were talking with Jesus. Then Peter said to Jesus, "Rabbi, it is good for us to be here; let us make three dwellings, one for you, one for Moses, and one for Elijah." (Mark 9:4-5)

Suddenly they saw two men, Moses and Elijah, talking to him. They appeared in glory and were speaking of his departure, which he was about to accomplish at Jerusalem. Now Peter and his companions were weighed down with sleep; but since they had stayed awake, they saw his glory and the two men who stood with him. Just as they were leaving him, Peter said to Jesus, "Master, it is good for us to be here; let us make three dwellings, one for you, one for Moses, and one for Elijah"—not knowing what he said. (Luke 9:30-33)

Here Moses and Elijah appear with Jesus, very much alive, and in the Luke account even "appearing in glory" and talking about upcoming events. The three disciples who witnessed this event perceived Moses and Elijah as being present in person with them—hence Peter's offer to make dwellings for all three figures: Jesus, Moses, and Elijah.

The natural conclusion for Christians who believe that human beings go to heaven right after death is that Moses and Elijah could appear with Jesus because they are not still in the grave awaiting resurrection, but are currently very much alive and well and living in heaven.

The thief on the cross

Of all the figures mentioned in the question, the incident of the thief on the cross comes closest to providing an explicit statement that a human being is now in heaven. Here is the full passage:

One of the criminals who were hanged there kept deriding him and saying, "Are you not the Messiah? Save yourself and us!" But the other rebuked him, saying, "Do you not fear God, since you are under the same sentence of condemnation? And we indeed have been condemned justly, for we are getting what we deserve for our deeds, but this man has done nothing wrong." Then he said, "Jesus, remember me when you come into your kingdom." He replied, "Truly I tell you, today you will be with me in Paradise." (Luke 23:39-43)

Here Jesus tells the repentant thief, who witnessed to his fellow thief on the cross, "Truly I tell you, today you will be with me in Paradise" (Luke 24:43).

Some have argued that since there is no punctuation in the original Greek, it could read instead, "Truly I tell you today, you will be with me in Paradise," so that it could refer to a future resurrection to Paradise. However, that would be a somewhat strained parsing of the Greek. That's why it is almost always translated in the traditional manner, as "today you will be with me in Paradise." In the list of all translations of Luke 24:43 at BibleGateway, though the specific wording varies, every single translation interprets the verse this way.

Though "Paradise" is not necessarily identical to "heaven," most Christians do interpret "Paradise," especially when it is mentioned in the New Testament, as a reference to heaven.

The saints who were resurrected during the crucifixion

Here is the relevant passage in its context:

Then Jesus cried again with a loud voice and breathed his last. At that moment the curtain of the temple was torn in two, from top to bottom. The earth shook, and the rocks were split. The tombs also were opened, and many bodies of the saints who had fallen asleep were raised. After his resurrection they came out of the tombs and entered the holy city and appeared to many. Now when the centurion and those with him, who were keeping watch over Jesus, saw the earthquake and what took place, they were terrified and said, "Truly this man was God’s Son!" (Matthew 27:50-54)

Here there is no explicit statement that these saints who were raised from the dead (or from having "fallen asleep," to use the Biblical term) went to heaven. It says only that they were raised, came out of the tombs, and entered the holy city—meaning Jerusalem—and appeared to many people there. However, the idea that they would then go back into their tombs after being resurrected seems unlikely to many Christians. So many Christians believe that they then went on to heaven.

Other Bible passages providing support for humans now being in heaven

As I mentioned earlier, none of these Bible passages about the figures specifically mentioned in the question explicitly and unequivocally say that these people are now in heaven. In fact, no matter what passages you care to quote, there are Christians who dispute that they point to humans currently being in heaven.

Having said that, here are a few other passages in the Bible that can easily be read as saying that there are humans currently residing in heaven.

First, two passages from the Gospels:

"And as for the resurrection of the dead, have you not read what was said to you by God, 'I am the God of Abraham, the God of Isaac, and the God of Jacob'? He is God not of the dead, but of the living." And when the crowd heard it, they were astounded at his teaching. (Matthew 22:31-33, and see the parallel passages in Mark 12:26-27; Luke 20:37-38)

Here Jesus uses the example of the words of God at the burning bush (in Exodus 3:6) in support of a resurrection. And the way he interprets God's words make it clear that he is speaking of Abraham, Isaac, and Jacob as being alive now, not still dead in their graves awaiting a future resurrection.

Another passage from the Gospels that speaks of human beings currently alive in the afterlife is Jesus' story of the rich man and Lazarus in Luke 16:19-31. Here is the most relevant part of the story:

The poor man died and was carried away by the angels to be with Abraham. The rich man also died and was buried. In Hades, where he was being tormented, he looked up and saw Abraham far away with Lazarus by his side. (Luke 16:22-23)

Here Jesus speaks of two men dying and immediately being carried to their place either with Abraham (commonly interpreted as heaven) or in Hades, the Greek word for the underworld. The entire action and dialogue of the story is presented as currently taking place in the afterlife. And even if this is read as a parable rather than as Jesus narrating a literal occurrence, his reference to these two people being immediately carried to their eternal dwelling place supports the idea of immediate resurrection of human beings to heaven (or hell) after death.

And two passages from the book of Revelation:

After this I looked, and there was a great multitude that no one could count, from every nation, from all tribes and peoples and languages, standing before the throne and before the Lamb, robed in white, with palm branches in their hands. They cried out in a loud voice, saying, "Salvation belongs to our God who is seated on the throne, and to the Lamb!" (Revelation 7:9-10)

Here John describes seeing a huge crowd of people from every nation standing before God's throne and praising God. And though the book of Revelation is commonly interpreted as referring to future events, John narrates it in the present tense, as things he was currently seeing in heaven. This is therefore another passage providing Biblical support for Christians to believe that humans go to heaven directly after death.

When he opened the fifth seal, I saw under the altar the souls of those who had been slaughtered for the word of God and for the testimony they had given; they cried out with a loud voice, "Sovereign Lord, holy and true, how long will it be before you judge and avenge our blood on the inhabitants of the earth?" They were each given a white robe and told to rest a little longer, until the number would be complete both of their fellow servants and of their brothers and sisters, who were soon to be killed as they themselves had been killed. (Revelation 6:9-11)

Here the book of Revelation speaks of souls slaughtered as pleading their case, receiving white robes, and being told to "rest," or wait, a while longer. This implies that they are currently alive in some state, which is most easily interpreted as their being alive in some part of heaven.

This is by no means an exhaustive list of passages providing Biblical support for the belief that there are humans in heaven now. But these are a few of the key passages related to the question.

Even more broadly, it has become a common belief among Christians that human beings become angels, and that this happens immediately after death. For the Biblical basis of this belief, see this Christianity.SE question: What is the biblical basis for humans becoming angels after they die?

On the origins of this belief in Christian history, see this Christianity.SE question: What is the source of the belief that the deceased become angels?